You might think everyone started off on the same page, but often there really isn't a "page" to rely on because, as applicable:

there wasn't a written contract;

the contract doesn't say anything at all about the problem that has arisen, leaving you totally without guidance on the controversy;

there is an agreement with an applicable clause, but the terms are ambiguous and the sides are reading it differently; or

the agreement the parties created, whether oral or written, is entirely unenforceable on a legal basis, voiding the entire instrument (not just the part governing the current point of contention) for one of a spectrum of reasons.

In each scenario we explored, there was a nagging certainty that an attorney's assistance was needed to protect rights and revenue. That help would have provided guidance in the case of unforeseen developments, or likely made them foreseeable in the first place.